Mesomorphism dependence on geometric shapes in isomeric molecules

Article abstract of DOI:10.1080/15421406.2016.1146931

A novel homologous series of symmetrical dimers RO-C₆H₄-CH?CH-COO-C₆H₄-OOC-CH?CH-C₆H₄-OR of liquid crystalline (LC) characteristics have been synthesized and studied with a view to understand and establish the effects of the molecular structure on the LC property or properties with special reference to geometrical shape in case of isomeric series. Homologous series consisted of 13 dimers (C₁ to C₁₈). C₁ to C₈ are only enantiotropic nematic and the rest of the dimer homologues from C₁₀ to C₁₈ are enantiotropically smectogenic in addition to nematogenic. Textures of the nematic phase are threaded or Schlieren and that of the smectic phase are of the type smectic A or C. Transition temperatures and the textures were determined by an optical polarizing microscopy equipped with a heating stage. Thermal stability for smectic and nematic are 161.0 °C and 237.6 °C, respectively, whose total mesophase (Sm+N) length ranges from 71.0 °C to 126.0 °C. Cr–M, Sm–N, and N–I transition curves of a phase diagram behaved in normal manner except negligible abnormality at C₁₄ dimer of N–I transition curve. N–I transition curve exhibited odd– even effect nearby up to C₇ dimer. Analytical and spectral data supported molecular structures of homologue dimers. Thus, present homologue dimer series is predominantly nematogenic and partly smectogenic and high-ordered melting type. LC properties of present novel series are compared with structurally similar series.

Full text of DOI:10.1080/15421406.2016.1146931

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on geometric shapes
in isomeric molecules
G. N. Bhola & U. C. Bhoya
To cite this article: G. N. Bhola & U. C. Bhoya (2016) Mesomorphism dependence on geometric
shapes in isomeric molecules, Molecular Crystals and Liquid Crystals, 630:1, 112-120, DOI:
10.1080/15421406.2016.1146931
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1146931
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Date: 01 July 2016, At: 19:04

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesomorphism dependence on geometric shapes in isomeric
molecules
G. N. Bhola and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot , Gujarat, India
ABSTRACT
KEYWORDS
Enantiotropic; liquid crystals;
nematic; smectic; symmetric
dimer
=
A novel homologous series of symmetrical dimers RO-C6H4-CH CH-
=
COO-C6H4-OOC-CH CH-C6H4-OR of liquid crystalline (LC) characteris-
tics have been synthesized and studied with a view to understand and
establish the effects of the molecular structure on the LC property or
properties with special reference to geometrical shape in case of iso-
meric series. Homologous series consisted of 13 dimers (C1 to C18). C1
to C8 are only enantiotropic nematic and the rest of the dimer homo-
logues from C10 to C18 are enantiotropically smectogenic in addition to
nematogenic. Textures of the nematic phase are threaded or Schlieren
and that of the smectic phase are of the type smectic A or C. Transition
temperatures and the textures were determined by an optical polar-
izing microscopy equipped with a heating stage. Thermal stability for
smectic and nematic are 161.0 °C and 237.6 °C, respectively, whose total
mesophase (Sm+N) length ranges from 71.0 °C to 126.0 °C. Cr–M, Sm–N,
and N–I transition curves of a phase diagram behaved in normal man-
ner except negligible abnormality at C14 dimer of N–I transition curve.
N–I transition curve exhibited odd– even effect nearby up to C7 dimer.
Analytical and spectral data supported molecular structures of homo-
logue dimers. Thus, present homologue dimer series is predominantly
nematogenic and partly smectogenic and high-ordered melting type.
LC properties of present novel series are compared with structurally sim-
ilar series.
Introduction
Liquid crystalline state (LC) [1] of a substance is well known for utility in the benefit of
mankind [2–7]. Among them, the LC dimers have played an important role and contribu-
tion [8–10]. Present investigation was planned to synthesize a homologous series of symmet-
ric dimers in which two mesogenic units are bonded to a phenyl ring to study the effect of
the molecular structure on LC properties of the dimer with special reference to the geomet-
rical shape of the isomeric dimers. Number of such dimers and monomer ester series with
LC properties have been reported till date [11–22]. The present study will include synthesis,
characterization, thermal properties, texture determination, and the comparison of some LC
properties with structurally similar dimer isomeric series. The characteristics of present ther-
motropic dimer series will be explained and interpreted in terms of molecular rigidity and
flexibility [23–30].
CONTACT G. N. Bhola
bholagn@gmail.com; U. C. Bhoya
Saurashtra University, Rajkot , Gujarat, India.
drucbhoya@gmail.com
Department of Chemistry,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
113
Scheme . Synthetic route to the series.
Experimental
Synthesis
4-Hydroxy benzaldehyde was alkylated by suitable alkylating agent R-X by refluxing for 4–
5 hr with acetone and K2CO3 by the established method [31]. 4-n-Alkoxy benzaldehydes were
individually condensed with malonic acid in pyridine in the presence of few drops of piperi-
dine to obtain trans 4-n-alkoxy cinnamic acids [32]. Transition temperatures of dimeric 4-n-
alkoxy cinnamic acids matched with the reported values. Trans n-alkoxy cinnamic acids were
individually treated with hydroquinone (1, 4 dihydroxy benzene) in the presence of DCC, 5
mol% DMAP and CH2Cl2 in a usual method [33]. Final products were washed with methanol
in vacuum filter then recrystallized till constant transition temperatures obtained. The syn-
thetic route to the series is mentioned in Scheme 1.
The chemicals 4-hydroxy benzaldehyde, R-X (alkyl halides), K2CO3, acetone, DCC,
DMAP, CH2Cl2, hydroquinone etc., were used as received except solvents which were dried
and distilled prior to use.
Characterization
Some selected members of a series were characterized by elemental analysis (Table 1), Infrared
1
spectroscopy, HNMR spectra and mass spectroscopy. Microanalysis was performed on

114
G. N. BHOLA AND U. C. BHOYA
Table . Elemental analysis for () Butyloxy () Hexyloxy () Dodecyloxy derivatives.
Elements % Found
H
Elements % Calculated
S. No.
Molecular formula
C
O
C
H
O



CHO
CHO
HO
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
C
EuroEA Elemental Analyzer. IR spectra were recorded on Shimadzu FTIR Model -IRAffinity-
1S (MIRacle 10), 1HNMR spectra were recorded on Bruker Avance-III (400 MHz) spectrom-
eter using CDCl3 as a solvent, Mass spectra were recorded on Shimadzu GC–MS Model No.
QP-2010. The liquid crystal behavior and the type of textures were determined by the misci-
bility method on microscopic observations.
Analytical data
Spectral data
HNMR in ppm for propyloxy derivative
0.95–0.99 (t, 6H,-CH3 of both -OC3H7 group), 3.85–3.91 (t, 4H,CH3-CH2-CH2-O- of both
alkoxy chain), 1.72–1.78 (m, 4H, CH3-CH2-CH2-O- of both alkoxy chain), 6.39–6.43 (d, 2H,
=
=
-OOC-CH CH- of both cinnamate group), 7.47–7.45 (d,2H, -OOC-CH CH- of both cin-
namate group), 7.12–7.34 (4H, middle phenyl ring), 6.87–7.73 (8H, both phenyl ring with
alkoxy chain). The NMR data are reliable with the molecular structure.
HNMR in ppm for pentyloxy derivative
0.92–0.95 (t, 6H,-CH3 of both -OC5H11 group), 1.28–1.34 (m, 4H,CH3-CH2-(CH2)3-O- of
both alkoxy chain), 1.38–1.43 (m, 4H, CH3-CH2-CH2-(CH2)2-O- of both alkoxy chain), 1.74-
1.78 (m, 4H, CH3-(CH2)2-CH2-CH2-O- of both alkoxy chain), 3.96–4.10 (t, 4H, CH3-(CH2)3-
=
CH2-O- of both alkoxy chain), 6.46–6.50 (d, 2H, -OOC-CH CH- of both cinnamate group),
=
7.60–7.64 (d,2H, -OOC-CH CH- of both cinnamate group), 7.39–7.54 (4H, middle phenyl
ring), 6.57–7.84 (8H, both phenyl ring with alkoxy chain). The NMR data are reliable with
the molecular structure.
IR in cm− for methyloxy derivative
3062 (C-H str. of aromatic), 2924 & 2846 (C-H str. of (-CH2-)n group of -OC6H13), 1705
=
=
(C O str. of carbonyl carbon of ester group), 1643 (C O str. of α,β unsaturated ketone),
=
=
1589 (C C str. of alkene), 1543 &1512 (C C str. of aromatic ring), 1450 (C-H bending of
–CH2 alkane group), 1381 & 1303 (C-H bending of alkene), 1249 (C-O str. of ester group),
1026, 987 & 702 (C-H bending of alkene disubstituted). The IR data are consistent with the
molecular structure.
IR in cm− for decyloxy derivative
3056 (C-H str. of aromatic), 2924 & 2846 (C-H str. of (-CH2-)n group of -OC10H21), 1720
=
=
(C O str. of carbonyl carbon of ester group), 1627 (C O str. of α,β unsaturated ketone),
=
=
1597 (C C str. of alkene), 1566 & 1504 (C C str. of aromatic ring), 1465 (C-H bending of
–CH2 alkane group), 1427 (C-H bending of alkene), 1288, 1249 & 1180 (C-O str. of ester

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
115
Table . Transition temperatures in °C.
Compound No.
n-alkyl chain CnHn+ (n)
Sm
N
Isotropic


























–
–
–
–
–
–
–
–































Sm-Smectic; N-Nematic
group), 1010, 995 & 825 (C-H bending of alkene disubstituted). The IR data are consistent
with the molecular structure.
Mass spectra of ethyloxy derivative
m/z (rel. int%): 458(M)+, 398, 316, 273, 245, 175 (base peak), 147, 119, 91
Mass spectra of heptyloxy derivative
m/z (rel. int%): 598 (M)+, 343, 245 (base peak), 218, 187, 147, 119, 91, 57
Results and discussion
1, 4 Dihydroxy benzene unit is a nonmesogenic unit (M.P. 172°C) which on condensation with
two symmetric mesogenic spacer units which yielded liquid crystalline symmetric dimers for
entire series from C1 to C18. Trans 4-n-alkoxy cinnamic acid acted as mesogenic units linked
to 1, 4 dihydroxy benzene unit. Mesomorphism commences from very first member of a series
as nematogenic character and contained up to last (C18) homologue dimer. However smec-
togenic property commenced from C10 homologue dimer to the last C18 dimer derivatives
in addition to nematogenic character. Transition temperatures as determined from an opti-
cal polarizing microscopy (Table 2) were plotted for the number of carbon atoms present in
n-alkyl chain of left and right n-alkoxy identical terminal end groups versus the transition
temperatures of homologue dimers C1 to C18. Cr–M, Sm–N, and N–I transition curves are
obtained on linking, like or related points as shown in a phase diagram of Figure 1. The Cr–
M transition curve continuously falls up to C7 dimer and rises in zigzag manner at the C8
dimer and then continuously descended to C18 homologue. Thus, it shows falling tendency
from C1 to C18 and behaved in a normal manner. The Sm–N transition curve rises from C10
to C12 and then passes through maxima at the C14 homologue dimer and then descended to
C18 homologue dimer through C16. Thus, it behaved in a normal expected manner, without
exhibition of the odd–even effect. The N–I transition curve continuously descended from C1
to C18 except C14, where negligible rise of one or two degree is observed. Thus, it behaved
in a normal manner with the exhibition of the odd–even effect in very very narrow range of
temperature. The odd–even effect diminished as series is ascended from and beyond a point
between C7 and C8 for homologue dimers of longer n-alkyl chain. The odd members of the N–
I transition curve occupy higher position than the even members of the transition curve. The
changing trend in mesogenic properties from homologue to homologue for the same series

116
G. N. BHOLA AND U. C. BHOYA
Figure . Phase Behavior of Series.
occurs because of changing the molecular length and the permanent dipole moment across
the long molecular axis, length to breadth ratio, ratio of the polarity to polarizability, suit-
able magnitude of anisotropic forces of intermolecular end to end or/and lateral attractions
as well as cohesive forces etc. also changes from homologue to homologue as a consequence
of molecular rigidity and flexibility of individual homologue, which varies accordingly and
hence the mesogenic tendency are differ from homologue to homologue in the same series.
The Sm–N transition curve is extrapolated to C8 nonsmectogenic [34–37] dimer homologue
to understand its nonsmectogenic behaviors. Thus, present homologous dimer series is pre-
dominantly nematogenic and partly smectogenic of high-ordered melting type.
High-ordered transition temperatures of homologue dimers are attributed to the molecular
rigidity possessed by three phenyl rings, two conjugated double bonds. Linearity of molecule
and high magnitude of length to breadth ratio, including the effect due to molecular flexibility
induced by left and right identical n-alkoxy terminal end groups, which causes suitable mag-
nitudes of anisotropic dispersion forces of intermolecular end to end and lateral attractions
and favorable intermolecular closeness. The highly polar n-alkoxy terminals on both sides
of a molecule maintains end-to-end attractions and molecular polarizibility throughout the
dimer homologous series which permits the all homologue molecules to float on the surface
with statistically parallel orientational order within definite range of temperature to exhibit
nematogenic character. However, on sequential addition of methylene unit or units from and
beyond C10 homologue, the intermolecular cohesion and closeness encourages lamellar pack-
ing of molecules in the preoccupied crystal lattices in rigid crystals, which allows sliding lay-
ered molecular arrangement under floating condition within definite range of temperature
to exhibit smectogenic character and then residual end-to-end dispersion forces and C-OR
dipolarities causes to induce nematogenic character from and beyond C10 to C18 homologue
dimers. The all molecules of all the homologues of a present dimer series are randomly ori-
ented in all possible directions from and beyond their isotropic temperatures. But on cooling

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
117
Figure . Structurally similar series.
the same, they show corresponding mesophase or mesophases in a reversible manner. The
appearance of mesophase or mesophases (smectic or/and nematic) is attributes to the favor-
able molecular rigidity and flexibility caused by the individual molecular structure of each
dimer of a novel series. The disappearance of the odd–even effect from and beyond the point
of merging the odd and even members N–I transition curves for longer n-alkyl chain of higher
homologues, is due to the coiling or bonding or flexing or coupling of n-alkyl chain with the
major axis of a core structure of a molecule. The unexpected positional status of the N–I curves
for odd and even members may be due to the presence of the exactly same -OR group, but the
difference of N–I transition points being very very small and the difference of their positional
status is less effective and negligible. Sm–N transition curve on extrapolation to nonsmecto-
genic C8 homologue merges into Cr–N transition point, which indicates that, smectogenic
mesophase length 137 − 137 = zero. Means there is no possibility of occurring the smectic
phase or lamellar packing of molecules in the crystal lattices of C8 homologue dimer. The
observed change in mesomorphic behaviors from homologue to homologue in the present
series is attributed to the sequential addition of methylene unit or units at the n-alkyl chain of
both terminal end groups in equal manner. However, the magnitudes of flexible n-alkyl chain
behavior and flexibility may differ and can cause variations in mesogenic behaviors and the
degree of mesomorphism. Thermal stability for smectic and nematic are 161.0°C and 237.6°C
attributed to their high magnitudes of molecular rigidity and intermolecular closeness. High
thermal resistivity yielded longer smectogenic and nematogenic mesophase lengths of the
order 21–85 °C and 08 −126 °C, respectively, and the total mesophase length ranges from 71
°C to 126 °C at the C18 and C7 homologue dimer, respectively. Some mesogenic properties of
present novel series of dimers are compared with the structurally similar other series-X [38]
as shown in Figure 2.
Homologous series-1 of dimers of present investigation and a homologous series-X of
dimers chosen for comparison are isomeric to each other with identical dihydroxy benzene
as linking unit but series-1 is constructed with 1,4 dihydroxy benzene and series-X is con-
structed with 1, 2 dihydroxy benzene with common two mesogenic units in both the isomeric
dimer series under comparison. Therefore, series-1 under discussion is linear lath like strati-
fied molecular structural shape, whereas dimer series-X has nonlinear ‘V’ shaped unstraitified
molecular structural shape. Thus, mesomorphic properties and the degree of mesomorphism
depend upon the differing features like molecular shapes. Consequently, it results into chang-
ing molecular rigidity and flexibility, dispersion forces, permanent dipole moment across the
long molecular axis, dipole–dipole interactions, C–OR bond dipolarity, molecular polarity
and polarizibility, intermolecular cohesion and closeness, etc., for the same homologue dimer
from series-1 to series-X.

118
G. N. BHOLA AND U. C. BHOYA
Table . Average thermal stabilities in °C.
Dimer series →

X
Smectic–Isotropic or Smectic–Nematic
Commencement of Smectic phase
. °C
(C–C)
C
. °C
(C–C)
C
. °C
(C–C)
C
Nematic–Isotropic Commencement of
Smectic phase
—
Total mesophase length range (Sm+N) in °C
. °C
(C) to
. °C (C) to
. °C (C)
. °C (C)
Isomeric homologous series-1 and X under comparative study differs with respect to geo-
metrical shape and size which causes difference in the mesogenic behaviors as mentioned in
Table 3.
Characteristics of phase behaviors are as under
r
Homologous series-1 is linear in which two arms of same mesogenic units are bonded to
1,4 position (180°) of nonpolar phenyl ring; whereas homologous series-X is nonlinear
‘V’ shaped in which exactly same as above two mesogenic arms are bonded to 1,2 position
of a phenyl ring at 60° angle.
Homologous series-1 is smectogenic in addition to nematogenic, whereas series-X is
only smectogenic without exhibition of nematogenic property.
Smectogenic mesophase commences from C10 homologue of a series-1, whereas it com-
mences early from C7 member of a series-X.
Nematic mesophase commences from very first member of a series-1, but it does not
commence till the last (C18) homologue member of a series-X.
Thermal stability for smectic for series-1 and X are 161°C and 95.95°C, respectively.
Thermal stability for nematic of series-1 is 237.6 oC, whereas nematic mesophase did
not facilitate at all for series-X.
Odd–even effect is exhibited by M–I (N–I or Sm–I) transition curve in both the series
r
r
r
r
r
r
under comparison.
r
Total mesophase length (Sm+N) range of novel dimer series-1 is much greater than a
series-X under comparison.
The polarity of chemical bond between two atoms depends upon the distribution of elec-
tron clouds concentrations on each end of a chemical bond, as a result of the difference of elec-
tronegativities of two atoms constituting a chemical bond. Therefore, greater the difference
of electronegativities, the greater will be the bond polarity. Thus, a chemical bond between
unlike atoms (e.g., H–Cl) will be more polar than a chemical bond between like atoms. Thus,
a chemical bond between unlike atoms will be polar, whose magnitudes of polarity will depend
upon the difference of electronegativity of individual atoms forming a chemical bond. Now,
the polarity of a molecule is a vector sum (not algebraic sum) of the polarities of all bonds in a
molecule, depending upon the angle between the bonds or a geometrical shape. The molecules
of series-1 in which two mesogenic units of ester derived from trans cinnamic acid, bonded
to nonpolar phenyl unit linearly at angle 180°, exactly in opposite directions. Therefore, all
bonds of a mesogenic unit whose vector sum of polarities of all bonds will be equal in mag-
nitudes and opposite in direction, for exactly same second mesogenic unit. Thus, the vector
sum of polarities due to two mesogenic units will cancel to each other. Moreover, nonpolar

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
119
phenyl ring will not contribute to total molecular polarity due to para substitution of meso-
genic units. Hence, total polarity of the molecules of series-1 are almost near to zero, whereas
the total polarity of the molecules of ‘V’ shaped series-X has the resultant value of molecular
polarity due to 1, 2 substitution of the same two mesogenic units bonded to ortho substituted
dihydroxy phenyl ring at 60° angle, i.e., series-X is more polar, polarizable and noncoplaner
than an isomeric series-1 of present investigation. Moreover, the intermolecular distance as
related to intermolecular closeness and suitable magnitudes of anisotropic dispersive forces of
end to end as well as lateral attractions, causing occurrence of smectic and/or nematic phase,
differs to a considerable extent due to the difference of the geometrical shapes for the same
homologue from series-1 to series-X. Thus, molecular rigidity and/or flexibility including the
positional status of the n-alkyl chains of n-alkoxy terminal end groups of series-1 and X dif-
fers. Consequently, the mesophase behaviors like commencement of smectic and/or nematic
phase, facilitating of only smectic or smectic plus nematic phase, thermal stabilities for smec-
tic and nematic, degree of mesomorphism or thermal resistivity against externally exposed
thermal vibrations, facilitation of lamellar packing of molecules in their rigid crystal lattices,
thermodynamically enthalpy (ꢀH) values, the extent of noncoplanarity of the molecules etc.
for the same homologue from series to series and from homologue to homologue in the same
series undergo variations as observed in Table 3. Hence, the mesogenic behaviors of isomeric
homologue dimer series vary with the geometrical shapes of the molecular structure.
Conclusions
r
Presently investigated novel dimer series is predominantly and entirely nematogenic and
partly smectogenic whose mesophase length is relatively higher and of high melting type.
The group efficiency order derived on the basis of (a) thermal stability (b) commence-
ment of mesophase and (c) mesophase length range for smectic and nematic are as under.
(a) Smectic: Linear series-1> ‘V’ shaped series-X
r
Nematic: Linear series-1> ‘V’ shaped series-X
(b) Smectic: ‘V’ shaped series-X > Linear series-1
Nematic: Linear series-1 > ‘V’ shaped series-X
(c) Sm+N: Linear series-1 > ‘V’ shaped series-X
r
Mesomorphism in isomeric series is very sensitive and susceptible to molecular shapes
of molecular structure.
Present novel investigation may be useful to the scientists and technologist working in
LC applications.
Thus, present investigation supports and raised credibility to the conclusions drawn ear-
lier.
r
r
Acknowledgments
The authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP
Sponsored), Saurashtra University, Rajkot, for research work. Authors are also thankful to Dr. A. V.
Doshi, Ex. Principal M.V.M. Science and Home Science College Rajkot, for his valuable co-operation
during present investigation as and when needed. Also thanks to the National Facility for Drug Discov-
ery through New Chemical Entities (NCE’s) for analysis of samples.
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